Normally a nitride based semiconductor light emitting device has a stack structure of a buffer layer, an n-type semiconductor layer, an active layer, a p-type semiconductor layer and an electrode on a substrate. The active layer is a region where electrons and holes are recombined, and has a structure that a quantum well layer is disposed between quantum barrier layers. Depending on kinds of material forming the active layer, the emitting wavelength of the nitride based semiconductor light emitting device is determined.
A single quantum well (SQW) structure having a single quantum well layer, a multi quantum well (MQW) structure having a plurality of quantum well layers, and a superlattice (SL) structure are known as the active layer. Among them, the active layer of a multi quantum well structure has been positively used since having excellent luminous efficiency per current and higher emitting output in comparison with a single quantum well structure.
Basically the luminous efficiency of such a nitride based semiconductor light emitting device depends on a recombination probability, i.e., internal quantum efficiency, of electrons and holes involved in a light-emitting process within the active layer. In order to enhance such internal quantum efficiency, a way of improving the structure of the active layer itself and a way of increasing the number of electrons and holes involved in a light-emitting process have been mainly studied.
A nitride based semiconductor light emitting device (also referred to as an ultraviolet light emitting device) that emits ultraviolet light with emitting wavelength of 365 nm or less has significantly poor luminous efficiency in comparison with other nitride based semiconductor light emitting devices having other wavelength. This results from the active layer having an AlGaN thin layer that contains high-concentration Al composition. Unfortunately, in an AlGaN/AlGaInN multi quantum well structure used as a light-emitting layer, a bending of energy band is caused by a difference in lattice constant between the quantum barrier layer of AlGaN and the quantum well layer of AlGaInN. This invites a disagreement between a spatial distribution of a wave function of electrons formed in the conduction band and a spatial distribution of a wave function of holes formed in the valance band. Therefore, the number of electrons and holes involved in a light-emitting process is reduced, and the luminous efficiency of the ultraviolet light emitting device is deteriorated.